Connetcting Vertices While Retopologizing
There are dozens of ways you can create the 4th vertex and connect a face when you have already 3, but most of them will be slow for retopo.

To work fast you want to enable the F2 addon (and while you are at it also have a look at BSurfaces):



With this your F shortcut gains new features, these ones help your case:


Make polygon by selecting corner vertex and hitting F:



Then you need to snap them to highpoly mesh. The addon has Auto Grab option which is great for this case - it let's you place that 4th vertex precisely.
Make polygon strips from an edge (extrude all the needed vertices with E, select edge, hit F couple times):




The F2 addon takes into account the position of your mouse cursor, when there is no clear direction where to put new face from the surrounding geometry: